B.5.4.6 Deleting Rows from Related Tables

If the total length of the
DELETE statement for
related_table is more than 1MB (the default
value of the
max_allowed_packet system
variable), you should split it into smaller parts and execute
multiple DELETE statements. You
probably get the fastest DELETE
by specifying only 100 to 1,000
related_column values per statement if the
related_column is indexed. If the
related_column isn't indexed, the speed is
independent of the number of arguments in the
IN clause.